Neuroscientists have uncovered genes underlying the fear of bullies in rodents. Mice constantly menaced by larger mice become loners, scared even of more docile mice. Bullying evidently triggers an upheaval in gene expression within the so-called mesolimbic dopamine pathway, a brain circuit linked with feelings of reward and desire. The result was the boosted expression of 309 genes and a stifling of 17, with many of these changes persisting for weeks afterward. A key regulator of the pathway is brain-derived neurotrophic factor, a chemical linked with antidepressant activity. Olivier Berton of the University of Texas Southwestern Medical Center and his colleagues knocked out the circuit's production of this factor, thereby reversing most of the bullying-inspired gene expression. The mice then accepted strangers despite repeated harassment. These findings, in the February 10 Science, could help treat depression and post-traumatic stress disorder, of which social withdrawal is a prominent symptom.